Of course, it's also possible that this discrepancy is of little importance. For one thing, it's already been noted by many pundits that many GOP campaigns are already writing off Iowa as a Romney win.
If that's the case, it makes sense to put your resources in other states (such as New Hampshire, South Carolina, etc.). ... Sadly, Martin writes that this disparity is also to be found in those states.
Of course, with the exception of Hillary Clinton's campaign, the other Dems have also agreed to stay out of Michigan. I think it's a safe bet that there are more Republican staffers on the ground in Michigan, and that's a far more important General Election state.
Still, this sounds a bit like the canary in the mine ...